Jazz, a musical form that grew out of a cross-fertilization of folk blues, ragtime, and European music, particularly band music, has been called the first art form to develop in the United States of America.

Though jazz has its folk roots, it was partly created by formally-trained musicians like Lorenzo Tio. Scott Joplin, who played ragtime piano in a brothel while writing an opera, shows the different influences at work in the period.

An important event in the development of jazz was the tightening of the Jim Crow (racial segregation) laws in Louisiana in the 1890s. Accomplished musicians of mixed race were no longer allowed to work with whites but were easily able to find work in black bands and orchestras, to which they applied conservatory standards.

There was a general liberalization of customs before World War I. Public dance halls, clubs, and tea rooms opened in the cities, and black dances like the cakewalk and the shimmy were eventually adopted by a white public, especially the flappers. White audiences saw them first in vaudeville shows, then performed by exhibition dancers in the clubs.

Much of the music for this dancing was not jazz, but it was new, and the fashion for new music did involve enthusiasm for some idea of jazz. Popular composers like Irving Berlin made attempts at jazzy writing, though they seldom used the specific musical devices that were second nature to jazz players--the rhythms, the blue notes. Nothing did more to popularize the idea of jazz than Berlin's hit song of 1911,"Alexander's Ragtime Band," which became a craze as far from home as Vienna. Although the song wasn't written in rag time, the lyrics describe a jazz band, right up to jazzing up popular songs, as in the line, "If you want to hear the Swanee River played in ragtime . . ."

Phonograph records made new music available everywhere. Through a few recordings aimed at black audiences, Louis Armstrong made the first decisive change in jazz. He played with the usual New Orleans march combo, in which everyone improvised simultaneously. But he was an extraordinary improviser, capable of creating endless variations on the initial melody. Musicians imitated him, not the ensemble, and jazz became a solo form.

The presence of dance venues influenced jazz musicians in two ways. They were more of them, since they could make a living, and jazz--like all the popular music of the 1920s--adopted the 4/4 beat of dance music.

With prohibition, the constitutional amendment that forbade the sale of alcoholic beverages, the legal saloons and cabarets were closed, but in their place hundreds of speakeasies appeared, where patrons drank and were entertained by musicians. The music was still a mixture of things--current dance numbers, novelty songs, show tunes. "Businessman's bounce music," as one horn player put it. But musicians with steady jobs, playing with the same companions, were able to go far beyond that. The Ellington band at the Cotton Club and the various Kansas City groups that became the Count Basie band date from this period.

The early development of jazz was racially segregated, reflecting the culture of the United States at the time, with the innovation of mainly black club musicians being taken onto bandstands by white band leaders, who tended to mould the music more to orthodox rhythms and harmony. The slow dissolution of this segregation began in the mid-1930s when Benny Goodman hired pianist Teddy Wilson, vibraharpist Lionel Hampton, and guitarist Charlie Christian to join small groups. In the mid to late 1930s the popularity of swing music and big band music was at its height, making stars of such men as Glenn Miller and Duke Ellington. Swing, the popular music of its time, covered a broad spectrum from "sweet" to "hot" bands, with the jazz content varying across the range.

A development of swing known as the jump blues anticipated rhythm and blues and rock and roll in some respects. It involved a use of small combos instead of big bands and a concentration on up-tempo music using the familiar blues chord progressions. One brief variation, known as boogie woogie, used a doubled rhythm--that is, the rhythm section played "eight to the bar," eight beats per measure instead of four. Big Joe Turner, a Kansas City singer who worked in the 1930s with swing bands like Count Basie's, became a boogie woogie star in the 1940s and then in the 1950s was one of the first innovators of rock and roll, notably with his song "Shake, Rattle and Roll".

The next major stylistic turn came with bebop, led by such distinctive stylists as the saxophonist Charlie Parker (known as "Bird"). This marked a major shift from music for dancing towards an intellectual art form of the first rank. Hard bop was an attempt to make bop more appealing to audiences by incorporating influences from soul music, gospel music, and the blues. Later bebop and hard bop musicians, such as trumpeter Miles Davis made more stylistic advances with modal jazz where the harmonic structure of pieces was much more free than previously, and frequently only implied by skeletal piano chords and bass parts. The instrumentalists would the improvise around a given mode of the scale. Soul jazz was a development of hard bop which centred on the Hammond organ.

Since then the stylistic diversity of jazz has shown no sign of diminishing, absorbing influences from such disparate sources as world music and avant garde classical music, including African rhythm and traditional structure, serialism and the extensive use of chromatic scale, by such musicians as Ornette Coleman or John Zorn. However, jazz's audience has shrunk dramatically and split somewhat, with a mainly older audience retaining an interest in traditional jazz, a small core of practitioners and fans interested in highly experimental modern jazz, and a constantly-changing group of musicians fusing jazz idioms with contemporary popular music genres, forming styles like acid jazz which contains elements of 1970sdisco, acid swing which combines 1940s style big-band sounds with faster, more aggressive rock-influenced drums and electric guitar.

Latin jazz deserves its own category or two - specifically there are two main styles of music that combine jazz harmonies and other concepts with rhythms and instruments from Africa and Latin America: Brazilian jazz and Afro-cuban jazz. While the music can be quite different, these forms of music are most definitely jazz, because they involve quite a bit of improvisation.

It's impossible to define exactly what jazz is, but nearly everyone would agree that a key element of jazz is improvisation. Musicians have been improvising since at least the time of Bach (who was a very accomplished improviser), but improvisation in a group setting was rare before jazz. The exact form of improvisation has changed over time. Early folk blues music was often based around a call and response pattern, and improvisation would factor into the lyrics, the melody, or both. Part of the Dixieland style involves musicians taking turns playing the melody while the others make up counter lines to go with it. By the swing era, big bands played carefully arranged sheet music, but the music would often call for one member of the band to stand up and play a short improvised solo. Finally, in Bebop improvisation takes center-stage, as almost the entire focus of the music is on clever improvised solos, with little attention given to the melody, or "head", of each piece.

When jazz musicians improvise, they usually use a chord progression - the series of chords that define the harmonic structure of a piece of music. For example, the Charlie Parker composition "Now's the Time" is 12 bars long and follows what Jazz musicians call a "twelve bar blues" progression. After the melody, the rhythm section keeps playing the same 12 bars of music while each soloist in turn improvises new melodies within the harmonic structure of the chords. Even if you are unfamiliar with music theory, you can get an idea of what's going on by humming the melody to yourself while you listen to the solo. This will make it more clear that the improvised melody is closely related to the chord progression of the piece. Fitting an improvised melody to the harmony is known as "playing the (chord) changes". As previously noted, later styles of jazz, such as modal jazz abandoned the strict notion of a chord progression, allowing the individual musicians to improvise more freely within the context of a given scale or mode.